The present invention relates to a method and apparatus for drying substrates.
Substrate drying apparatus are used in the manufacture of integrated semiconductor circuits, display screens comprising liquid crystals, electrical circuits on boards or synthetic material, and video or phonorecords. In these processes, substrates of semiconductor, ceramic, and polymer materials are treated with liquids, for example, by immersion in etchant vats for etching the substrate, developing vats for developing exposed photoresist layers on the substrate, galvanic vats for deposition of metals on the substrate, or rinsing vats for cleaning the substrates. After treatment in the liquid, the thin film of liquid residue that remains on the substrate is removed by a drying process.
Several conventional methods are used to dry the substrate. The centrifuge or "spin-dry" method uses a centrifuge which is rapidly rotated about an axis transverse to the surface of the substrate. Centrifugal force generated by the rapid rotation forcibly ejects most of the residual liquid on the substrate. However, conventional centrifuging apparatus and processes have several disadvantages. First, evaporation of the thin film of water left on the substrate after high speed rotation of the substrate can cause spotting and streaking due to small amount of nonvolatile matter dissolved in the water. Furthermore, centrifuging methods often leave contaminant residue on textured substrate surfaces having features or holes that capture the residue.
An improved centrifuging process described in U.S. Pat. No. 5,271,774, to Leenaars, et al., flows a vapor that is miscible with liquid on the substrate surface, during the centrifuging process, to reduce the contaminants left on the substrate. The vapor mixes with the liquid residue on the substrate to form a solution having a lower surface tension that is more easily ejected from the substrate during centrifuging to leave behind less contaminant residue. However, even the improved centrifuging processes and apparatus are complex and prone to breakdown due to mechanical problems resulting from the high speed rotation of the substrate. Because the centrifuge spin dryers rely on centrifugal force to "throw" water off the wafer surfaces, mechanical stresses often break or damage large or thin substrates. Also, moving parts inside the centrifuge erode to form contaminant particles which deposit on the substrate. Furthermore, static electricity formed on the substrate surface during high speed rotation of the substrate in the dry atmosphere can attract oppositely charged airborne contaminant particles onto the substrate.
A preferred method for drying advanced VLSI and ULSI integrated circuit substrates uses the Marangoni principle, which holds that fluid flows from lower to higher surface tension regions across a fluid surface. One type of Marangoni dryer operates by immersing a substrate in a tank containing liquid and flowing miscible vapor over the liquid. Liquid added from the bottom of the tank overflows over the walls of the tank. The continuously overflowing liquid results in a non-uniform concentration of dissolved vapor, and resultant surface tension gradients, across the liquid surface. In conventional Marangoni dryers, the substrates are held in a cassette holder immersed in water, and the substrates are slowly lifted from the fluid using a complex robotic mechanism that pulls the substrates out from the tank. As the substrates are pulled out, residual liquid on the substrate flows off the substrate surface, in the direction of the higher surface tension regions on the liquid surface, thereby drying the substrate. The robotic pulling mechanisms are typically positioned in a lid that forms the top of the dryer chamber or under the substrates at the bottom of the dryer chamber or tank.
However, conventional Marangoni drying systems have several problems. The complex robotic pulling mechanisms used to pull out the substrates are prone to failure and have high maintenance costs. Also, the pulling device has mechanical parts which move and wear out seals. The worn parts form contaminant particles that deposit on the substrate. Furthermore, vibrations of the robotic pulling mechanism can disturb the flow of residual liquid off the substrate surface. Furthermore, where the robotic lifting mechanism touches portions of the substrates, residual liquid remaining at the contact points on the substrate surface causes streaks and stains to form on the substrate.
Yet another problem with conventional Marangoni drying systems arises because the inlet and exhaust for the vapor introduced into the chamber do not provide a uniform distribution of vapor across the liquid surface. In conventional systems, the flow rates and flow pattern of vapor across the surface of the liquid from the vapor inlet to the vapor exhaust change as a function of the height of the substrates or level of fluid in the reservoir. It is desirable to have a uniform flow of vapor across the surface of the fluid to provide more consistent drying of the liquid residue on the substrate surface.
Thus, it is desirable to have substrate drying apparatus and method that can remove residual liquid films on a substrate surface efficiently without leaving behind contaminants or stains. It is further desirable to have a drying apparatus that provides a uniform flow of vapor across the surface of the fluid in the apparatus. It is also desirable to control the level and flow rate of fluid in the tank to maintain uniform surface tension gradients across the liquid surface, particularly at the intersection of the surface of the substrate with the surface level of the drying fluid. It is further desirable for the apparatus and method to operate with low maintenance costs and to provide high processing throughput.